Aliphatic orthothioesters as rocket fuels



Sept. 5, 1961 D. R. CARMODY ET AL 3,

ALIPHATIC ORTHOTHIOESTERS AS ROCKET FUELS Filed Feb. 25, 1952 OX/D/ZER INVENTORS Don R. Garmady BY Alex Z/efz A TTORA/EY Patented Sept. 5, 1961 ice Filed Feb. @5, 1952, Ser. N 0. 273,272 8 Claims. (Cl. Gil-35.4)

This invention relates More particularly, it relates to novel fuels that are spontaneously combustible, when contacted with an oxidizer for the gerllfraltiitin of hlot gases in a rocket motor.

oc e propu sion is now being used to assi t 1n take-off or to attain bursts of speed in exie si o i ih i attainable with the regular power plant. Also rocket propulsion is being used in the military projectile field wherein an explosive container is air-borne by means of fin aLtached rocket motor; these projectiles may be 111; from the earths surface or from an airplane Rockets use either a single self-contained propellant-awhich may be either a solid or a ci r a separate fuel and a separate oxidizer--bipropellant. The bipropellants are stored in separate tanks outside the rocket motor itself. The bipropellant rocket motor consists of a suitable combustion chamber provided with one or more pairs of injectors adapted to spray therein the fuel and the oxidizer, separately and simultaneously. The combustion of the fuel and the decomposition of the oxidizer creates a mass of hot, burning gases which are e ected at high velocity through a suitable nozzle" 2%: reaction from this ejection provides the propulsive .The ox dizer may be oxygen, preferably liquid, hydrogen perox de, fuming nitric acid, nitrogen ltetroxide mixtures of nitric acid and sulfuric acid or oleum, etc. i

The ignition reaction between the fuel and the oxidizer may be Initiated by an electric spark, a hot wire, a hot surface or may be spontaneous. A spontaneous combustion or self-ignition is preferred because of the possibilrtres of electrical and mechanical failure of the spark and hot surface methods of ignition. A fuel which is self-igniting when contacted with an oxidizer is called a hypergolic material.

The walls of the combustion chamber become very hot from the heat of the burning gases generated by the reaction of the fuel and the oxidizer. This hot surface and the mass of hot gases in the chamber, has a pro nounced favorable eifect on the self-ignition characteristics of the fuel and oxidizer. Many fuels which are nonhypergolic at the temperature existing in the fuel tank of the rocket unit are rapidly hypergolic in the extremely hot combustion chamber. For economy of operation, a fuel that is hypergo-lic at very low temperatures may be used to initiate the combustion in and to start the cold reaction motor; the use of this starter fuel may be continued until the hot gases generated have heated the combustion chamber to a high temperature; at this point the flow of the starter fuel can be stopped and a cheaper, although not as highly hypergolic or even a non-hypergolic fuel can be utilized for the continuous operation of the reaction motor.

An object of this invention is reaction propulsion by means of a hypergolic fuel and a nitric acid oxidizer. Another object is to provide a hypergolic mixed fuel for rocket propulsion, which mixed fuel contains appreciable amounts of essentially non-hypergolic hydrocarbons. Still another object is to provide a fuel for reaction propulsion which is hypergolic at low temperatu e A P ticular object is a reaction propulsion method that is not to reaction propulsion.

dependent on auxiliary ignition devices for initiating combustion at low temperatures.

Very briefly, the novel hypergolic fuel of this invention consists of an aliphatic orthothioester having the formula composition R-C-(SR wherein: C represents the element carbon, S represents the element sulfur, R represents the same or different aliphatic radicals containing from 1 to 4- carbon atoms and R represents hydrogen or an aliphatic radical containing from 1 to 3 carbon atoms. A novel hypergolic mixed fuel is made by mixing the above defined orthothioester with an essentially non-hypergolic hydrocarbon in proportions that will be described later in this specification.

Nitric acid has been found to be among the best oxidizers for rocket propellant fuels. The most commonly used material is white fuming nitric acid-abbreviated WFNA- which normally contains less than about 2 weight percent of water. More dilute solutions have been utilized by fortifyin-g the acid with nitrogen tetroxide N 0 Red fuming nitric acid-RPNA-norrnally contains less than about 5% of Water and between about 5 and 2.0% of N 0 Sodium and potassium nitrites and and sodium and potassium nitrates are often added to WFNA to depress the freezing point; usually an aque ous solution of the salt is used. Liquid nitrogen tetroxide is an excellent oxidizer when used above its freezing point. An excellent oxidizer for use at temperatures as low as about -65 F. is obtained by adding 10 to 30% of sulfuric acid or about 1 to 30% of oleum to strong nitric acid. The particularly effective nitric acid oxidizers contain not more than about 5 weight percent of nonacidic material, such as, water or aqueous potassium. nitrate solution. The preferred oxidizers are nitric acidoleum mixtures, white fuming nitric acid and red fuming nitric acid. The use of the general term nitric acid oxidizer in this specification and in the claims is intended to include all the favorable compositions described in this paragraph.

It has been discovered that certain aliphatic orthothioesters have hypergolic properties with certain nitric acid oxidizers at temperatures as low as about 70 F. The commonly accepted structural formula for these orthothioesters is:

The symbols R, R" and R' in the above structural formula represent the same or different aliphatic radicals containing from 1 to 4 carbon atoms; the symbol R represents hydrogen or an aliphatic radical containing from 1 to 3 carbon atoms. The term aliphatic is intended to include radicals that contain one or more unsaturated linkages as 'well as the alkyl radicals. The preferred classes are the aliphatic orthothioformates and the aliphatic orthothioacetates. Individual members that are particularly effective are ethyl orthothioformate, ethyl or thothioacetate, n-propyl orthothioformate, n-butyl orthothioformate, allyl orthothioformate, allyl orthothioacetate and allyl orthothiopropionate.

The orthothioesters of this invention are in general heavy, mobile, high boiling liquids; they are resistant to attack by alkaline materials; they are susceptible to attack by acidic materials and are converted to nonhypergolic materials. The methyl and t-butyl orthothioformates have limited utility because the pure compounds have freezing points well above 0 F. The presence of minor amounts of impurities, formed in the preparation of the orthothioesters, is beneficial in that the freezing point of the impure material is markedly tween 0.01 and 0.2 ml. per ml. of oxidizer.

lower than the freezing point of the pure material. There appears to be no appreciable difference in hypergolic activityofthepure compound and of the compounds containing minor amounts of impurities. Those materials which contain minor amounts of impurities from side reactions in the preparation of the orthothioesters are included within the scope of the invention.

Ethyl orthothioformate was prepared by reacting anhydrous formic acid and ethyl mercaptan in the presence of anhydrous hydrogen chloride. The anhydrous formic ether and the residue was distilled. The product used in these tests distilled from 240 to 270 F. at 12 mm. and had a freezing point below --70 F.

It has been found that liquid hydrocarbons which are essentially non-hypergolic can be mixed with orthothioesters to obtain a fuel that is hypergolic with nitric acid oxidizer. The essentially non-hypergolic hydrocarbon should have a low freezing point, i.e., below 70 F., in order to obtain a low temperature hypergolic mixed fuel. The boiling point of the essentially non-hypergolic hydrocarbon has an effect on the ignition temperature of the mixed fuel; a non-hypergolic hydrocarbon with a maximum boiling point below about 600 F. is preferred. A particularly suitable non-hypergolic hydrocarbon is a virgin petroleum distillate boiling between about 100 and 525 F. and having an RVP of about 6 lbs.

The composition of the mixed hypergolic fuel is dependent primarily upon the particular nitric acid oxidizer being used and, below about F., the composition of the mixed fuel, which is still hypergolic, is substantially independent of temperature. When using nitric acid containing less than about of water as the oxidizer, such as, WFNA or RFNA, and an alkyl orthothioformate 'as the hypergolic fuel, as much as 20 volumes of nonbons, which mixed fuel can contain less of the orthothioesters than a mixed fuel containing essentially non-hypergolic hydrocarbons. Such mixed fuels are within the scope of the invention.

The ignition characteristics of fuels were studied using .a drop test method. This method utilizes a test tube,

1 in. x 4 in., containing 1 ml. of oxidizer. The fuel is added dropwise into the test tube by means of a syringe calibrated in 0.01 ml. markings. Usually 0.1 ml. of fuel is added per test; however, the feed usage may vary be- Low temperature tests were carried out by cooling the test tube and the oxidizer contained therein to the desired temperature by means of a Dry Ice-chloroform bath; a drying tube inserted into the top of the test tube excluded moisture. The fuel was cooled separately to the desired temperature. By supercooling, it was possible to carry out tests at temperatures below the freezing point of the oxidizer. The time elapsing between the addition of the fuel to the oxidizer and ignition thereofthe ignition delay--'-was determined visually as either: very short,

short, ignition and no ignition. A very short delay corresponds to substantially instantaneous ignition.

In order to measure more accurately the amount of fuel added and also to approach more closely a reproducible degree of mixing, a capillary tube test was also used. A capillary tube of 2 mm. diameter or less, with a syringe attached at one end, is filled with a measured amount of fuel undergoing the test; an air space is left at the end of the tube. The capillary tube is inserted into the oxidizer in a beaker and the fuel is injected into the acid by depressing the syringe plunger. By this capillary tube method, amounts of fuel on the order of 0.0002 ml. can be added to the oxidizer.

The following tests are illustrative only and are not intended to limit our invention.

Test 1 This series of runs used ethyl orthothioformate, prepared according to the method described above, as the fuel. The minimum hypergolic temperature and visual ignition delay were determined when using 0.1 ml. of

This series of runs was made in the capillary tube method at +75 F. to determine the minimum volume of fuel required for ignition with ml. of WFNA as the oxidizer.

Minimum volume Compound: m1.

Ethyl orthothioformate 0.013 Aniline 025 This invention is particularly advantageous when the fuel, oxidizer and combustion chamber are initially at atmospheric temperature as combustion begins without auxiliary ignition devices or without preheating of the combustion chamber. The nitric acid oxidizer and the fuel should be added to the combustion chamber separately and simultaneously so as to contact each other with considerable intermingling action. Usually the relative amounts of the two materials will be somewhat in excess of the theoretical oxygen balance ratio of oxidizer to fuel. When using the hypergolic fuels of this invention between about 3 to 5 pounds of WFNA are injected per pound of the fuel. While it is possible to vary the ratio during operation, it is preferred to maintain a constant ratio.

By way of example, this invention is applied to the driving of a ground-to-air missile. The figure shows a schematic layout of the combustion chamber and bipropellant feed system of a reaction motor, such as is used in a military rocket projectile. In the figure, vessel 11 contains a quantity of inert gas under high pressure;

nitrogen or helium is a suitable gas. Helium is passed through line 12, through a regulatory valve 13 which passes the helium into line 14 at a constant pressure. From line 14, the helium is passed into line 16 which is connected to the vessels containing the fuel and the oxidizer. Vessel 17 contains the oxidizer; the pressure of the helium from line 16 forces the oxidizer through line 18, through solenoid actuated throttling valve 21, through line 23, and through injector 24 into combustionchamber 26. Combustionchamber 26 is provided with a nozzle throat opening 28. Vessel 31 contains the fuel. The helium pressure forces the fuel out of vessel 31 through line 32, through solenoid actuated throttling valve 41, through line 43 and through injector 44 into combustion chamber 261 The injectors 24 and 44 are so arranged that the streams of liquid violently impinge and thoroughly intermingle and ignite. The combustion of the fuel and the oxidizer results in the generation of a large volume of very hot gases which pass out of the combustion chamber through throat 28; the reaction from this expulsion of gases drives the rocket.

Herein a mixture of 60%of a virgin distillate with a maximum boiling point of 590 F. and 40% of allyl orthothioformate is used as the fuel. The oxidizer is a mixture of WFNA and oleum. The missile is launched by activating the solenoids on valves 21 and 41. The oxidizer and the fuel are forced into the combustion chamber in the Weight ratio of 4.0 to 1. Instantly combustion takes place and the missile hurtles toward the target.

We claim:

1. A reaction propulsion method which comprises injecting separately and essentially simultaneously into a combustion chamber a hypergolic fuel consisting essentially of an orthothioester having a generic formula, RC(SR wherein C represents carbon, S represents sulfur, R represents a member selected from the class consisting of hydrogen and alkyl groups, said groups containing not more than 3 carbon atoms and R represents a member selected from the class consisting of alkyl groups and alkenyl groups, which groups contain not more than 4 carbon atoms and a nitric acid oxidizer which contains not more than 5 weight percent of nonacidic materials, said fuel and said oxidizer being injected in a ratio and at a rate suflicient to initiate a hypergolic reaction and to support combustion of the fuel.

2. The method of claim 1 wherein said oxidizer is selected from the group consisting of white fuming nitric acid, red fuming nitric acid and nitric acid-oleum mixtures.

3. The method of claim 1 wherein said fuel is ethyl orthothioformate.

4. A reaction propulsion method which comprises injecting separately and essentially simultaneously into a combustion chamber a nitric acid oxidizer containing not more than about 5 Weight percent of non-acidic materials and a hypergolic fuel consisting essentially of (a) a liquid hydrocarbon and (b) anorthothioester having the generic formula RC(SR wherein C represents carbon, S represents sulfur, R represents a member selected from the class consisting of hydrogen and alkyl groups, said groups containing not more than 3 carbon atoms and R represents a member selected from the class consisting of alkyl groups and alkenyl groups containing not more than 4 carbon atoms, said fuel and said oxidizer being injected in a ratio and at a rate sufficient to initiate a hypergol-ic reaction and to support combustion of the fuel.

5. The method of claim 4 wherein said hypergolic fuel contains not more than about 20 volume percent of an essentially non-hypergolic liquid hydrocarbon boiling below about 600 F.

6. The method of claim 5 wherein said oxidizer consists essentially of a mixture of nitric acid and oleum.

7. The method of claim 4 wherein said fuel consists of about volume percent of a virgin petroleum distillate boiling over the range of about and 590 F. and the remainder consisting essentially of allyl orthothioformate.

8. A reaction propulsion method which comprises injecting separately and essentially simultaneously into a combustion chamber a hypergolic file] consisting essentially of allyl orthothioformate, and a nitric acid oxidizer which contains not more than 5 weight percent of nonacidic materials, said fuel and said oxidizer being injected at a ratio and at a rate suflicient to initiate a hypergolic reaction and to support combustion of the fuel.

References Cited in the file of this patent UNITED STATES PATENTS 2,229,65'1 Hanford et a1. Jan. 28, 1941 2,389,153 Kendall Nov. 20, 1945 2,489,051 Sayward et a1. Nov. 27, 1949 2,493,234 Farkas Jan. 3, 1950 

1. A REACTION PROPULSION METHOD WHICH COMPRISES INJECTING SEPARATELY AND ESSENTIALLY SIMULTANEOUSLY INTO A COMBUSTION CHAMBER A HYPERGOLIC FUEL CONSISTING ESSENTIALLY OF AN ORTHOTHIOESTER HAVING A GENERIC FORMULA, R-C-(SR**X)3 WHEREIN C REPRESENTS CARBON, S REPRESENTS SULFUR, R REPRESENTS A MEMBER SELECTED FROM THE CLASS CONSISTING OF HYDROGEN AND ALKYL GROUPS, SAID GROUPS CONTAINING NOT MORE THAN 3 CARBON ATOMS AND R**X REPRESENTS A MEMBER SELECTED FROM THE CLASS CONSISTING OF ALKYL GROUPS AND ALKENYL GROUPS, WHICH GROUPS CONTAIN NOT MORE THAN 4 CARBON ATOMS AND NITRIC ACID OXIDIZER WHICH CONTAINS NOT MORE THAN 5 WEIGHT PERCENT OF NONACIDIC MATERIALS, SAID FUEL AND SAID OXIDIZER BEING INJECTED IN A RATIO AND AT A RATE SUFFICIENT TO INITIATE A HYPERGOLIC REACTION AND TO SUPPORT COMBUSTION OF THE FUEL. 